1. Field of the Invention
The present disclosure relates to certain plant genes implicated in oil production and the use of such genes to boost or to modulate oil content in plant seeds. More particularly, the present disclosure relates to modulation of the ratio between different fatty acids in a plant or parts thereof through controlled expression of these genes.
2. Description of the Related Art
Plant oils are used in a variety of applications. Plants that synthesize and store large amounts of oils in their seeds are an important source of oils for both human and animal consumption. Plant oils are also widely used in various industries, such as in the paint and coating industry. Depending upon the intended use, different fatty acid compositions in plant oils are desired.
Plant and animal oils (or fats) are composed almost entirely of triacylglycerols (TAGs). TAG is an ester formed by fatty acids and glycerol where the fatty acids are esterified to the three hydroxyl groups of glycerol. Plant and fish fat sources are generally considered more healthy than fat sources from most animals because these plant and fish fat sources typically have higher content of unsaturated fatty acids. The intake amount of unsaturated fatty acids and the ratio between these fatty acids in the food sources have certain causal relationship with the incidence of cardiovascular and other chronic diseases in humans. For review, see Simopoulos, A P, Exp Biol Med (Maywood), 233(6):674-88 (2008). Moreover, saturated fatty acids typically have higher melting points than unsaturated fatty acids and are thus less desirable in many applications. For instance, when used as a fuel, saturated fatty acids may cause clouding at low temperatures, and may confer poor cold flow properties to the fuel.
Long chain polyunsaturated fatty acids are essential components of phospholipids in the cell membrane and are also precursors of various metabolites which are important in various functions of the human body. Certain long chain polyunsaturated fatty acids can be synthesized in humans by the “omega-6 pathway” from dietary omega-6 and omega-3 fatty acids, e.g., linoleic acid (also referred to as LA or 18:2) and α-linolenic acid (also referred to as ALA or 18:3), respectively. However, because humans cannot synthesize LA or ALA, these fatty acids have to be obtained from diet or other sources.
Because omega-6 and omega-3 fatty acids each play an important yet distinct role in the human body, it is important to maintain an optimal balance between these two types of unsaturated fatty acids. Although a ratio of 5:1 has been recommended for omega-6 and omega-3 fatty acids in human diet, the ratio has shifted heavily toward omega-6 fatty acids in the current western diet. Sargent, J. R. (1997) Br. J. Nutr. 78, Suppl. 1, 5-13. Indeed, the amount of omega-6 fatty acids is, by some estimates, up to 30-fold higher than the recommended value. This skewed ratio between omega-6 and omega-3 fatty acids is at least partially attributable to the consumption of increasing amount of vegetable oils and other foods that are rich in LA, but low in ALA. Simopoulos, A. P. (1999) Am. J. Clin. Nutr. 70, 560S-569S. Thus, increasing the amount of ALA in common food sources such as plant oils may help correct the skewed ratio between dietary omega-6 and omega-3 fatty acids.
TABLE 1Characteristics of the major Fatty AcidsCarbons: Double BondsNameSaturation16:0Palmitic AcidSaturated18:0Stearic AcidSaturated18:1Oleic Acidmonounsaturated18:2Linoleic Acidω-6 polyunsaturated18:3α-Linolenic Acidω-3 polyunsaturated
Table 1 summarizes major fatty acids in plants. The designations (18:1), (18:2), (18:3), etc., refer to the number of carbon atoms in the fatty acid chain and the number of double bonds therein. As commonly used in the field and in this disclosure, the designations sometimes take the place of the corresponding fatty acid common name. For example, oleic acid (18:1) contains 18 carbon atoms and 1 double bond, and may be referred to as “18:1”.
In higher eukaryotes, cytochrome b5 (Cb5) is a small heme-binding protein typically associated with the endoplasmic reticulum (ER) and the outer mitochondrial membranes. Cb5 provides electrons in desaturation of acyl CoA fatty acids (FAs) [1]. In higher plants, Cb5 has also been implicated as electron donor in hydroxylation of acyl CoA FAs [2-4], divergent FAD2 such as conjugase, acetylenase mediated reactions [5-7], desaturation and hydroxylation of sphingolipids [8, 9], sterol desaturations [10] and cytochrome P450 reactions [11]. Apart from its role in lipid metabolism associated reactions, recently it has been reported that interaction of ER resident Cb5 with plasma membrane associated sucrose and sorbitol transporters results in up-regulation of their affinity to their substrate sugars which is critical for adjusting sugar level in cells [12].
Higher plants such as tobacco [16,17], Arabidopsis [4,18], Tung [19], Crepis alpina [7], and soybean are uniquely endowed with multiple Cb5 genes as opposed to a single one in mammals [20] and yeast [21], respectively. Apart from the typical ER or outer mitochondrial membrane associated “independent” Cb5, the Cb5 like domain has been reported in many front-end desaturases, such as Δ5 or Δ6 desaturase of mammals and C. elegans [8] and Δ6-desaturase of borage plants [22]. The Cb5 motif has also been reported in higher plant sphingolipid Δ8-LCB desaturase [8] and nitrate reductase [23]. The extensive molecular-genetic and biochemical characterization of higher plant FAD2 and FAD3 in the past has led to significant advances in our understanding of 18C PUFAs synthesis; however, parallel information regarding relationship of various Cb5 isoforms in such desaturases is limited.